Try describing the color without addressing it flat out. Establishing dark skin is important for an initial introduction, but then describe how their skin reacts to the environment, or how their hair is styled. Darker skin, especially more melinated skin tends to flash light against it more. There are also a lot of variations to darker skin due to hyper pigmentation. Addressing their hairstyle can also make things more obvious. I have done this before by describing how a character in one of my projects styled her dreads.
Most white characters aren't outright described as white, but their other features are paid more attention to. That can be done for characters of color as well, but with the different features certain races and ethnicities have. It can really improve your writing anyway, as describing how "the glow of the sun glittered across her cheek, cutting a streak across her darkened skin," sounds more interesting than, "she had a deep, caramel coating to her skin in the sun."